802 research outputs found
The impact of enhanced He and CNONa abundances on globular cluster relative age-dating methods
The impact that unrecognised differences in the chemical patterns of Galactic
globular clusters have on their relative age determinations is studied. The two
most widely used relative age-dating methods, horizontal and vertical, together
with the more recent relative MS-fitting method, were carefully analyzed on a
purely theoretical basis. The BaSTI library was adopted to perform the present
analysis. We find that relative ages derived using the horizontal and vertical
methods are largely dependent on the initial He content and heavy element
distribution. Unrecognized cluster-to-cluster chemical abundance differences
can lead to an error in the derived relative ages as large as ~0.5 (or ~6 Gyr
if an age of 12.8 Gyr is adopted for normalization), and even larger for some
extreme cases. It is shown that the relative MS-fitting method is by far the
age-dating technique for which undetected cluster-to-cluster differences in the
He abundance have less impact. Present results are used in order to pose
constraints on the maximum possible spread in the He and CNONa elements
abundances on the basis of the estimates - taken from the literature - of the
Galactic globular clusters relative age dispersion obtained with the various
relative age-dating techniques. Finally, it is shown that the age-metallicity
relation found for young Galactic globular clusters by the GC Treasury program
is a real age sequence and cannot be produced by variations in the He and/or
heavy element distribution.Comment: 26 pages, 8 figures, accepted for publication in ApJ
The red giant branch phase transition: Implications for the RGB luminosity function bump and detections of Li-rich red clump stars
We performed a detailed study of the evolution of the luminosity of
He-ignition stage and of the red giant branch bump luminosity during the red
giant branch phase transition for various metallicities. To this purpose we
calculated a grid of stellar models that sample the mass range of the
transition with a fine mass step equal to . We find that for
a stellar population with a given initial chemical composition, there is a
critical age (of 1.1-1.2~Gyr) around which a decrease in age of just 20-30
million years causes a drastic drop in the red giant branch tip brightness. We
also find a narrow age range (a few yr) around the transition,
characterized by the luminosity of the red giant branch bump being brighter
than the luminosity of He ignition. We discuss a possible link between this
occurrence and observations of Li-rich core He-burning stars.Comment: 5 pages, 4 figures, Astronomy & Astrophysics in pres
On the Estimation of Systematic Uncertainties of Star Formation Histories
In most star formation history (SFH) measurements, the reported uncertainties
are those due to effects whose sizes can be readily measured: Poisson noise,
adopted distance and extinction, and binning choices in the solution itself.
However, the largest source of error, systematics in the adopted isochrones, is
usually ignored and very rarely explicitly incorporated into the uncertainties.
I propose a process by which estimates of the uncertainties due to evolutionary
models can be incorporated into the SFH uncertainties. This process relies on
application of shifts in temperature and luminosity, the sizes of which must be
calibrated for the data being analyzed. While there are inherent limitations,
the ability to estimate the effect of systematic errors and include them in the
overall uncertainty is significant. Effects of this are most notable in the
case of shallow photometry, with which SFH measurements rely on evolved stars.Comment: 28 pages, 9 figures, ApJ in pres
Impact of Rotation on the Evolution of Low-Mass Stars
High precision photometry and spectroscopy of low-mass stars reveal a variety
of properties standard stellar evolution cannot predict. Rotation, an essential
ingredient of stellar evolution, is a step towards resolving the discrepancy
between model predictions and observations. The first rotating stellar model,
continuously tracing a low-mass star from the pre-main sequence onto the
horizontal branch, is presented. The predicted luminosity functions of stars of
globular clusters and surface rotation velocities on the horizontal branch are
discussed.Comment: 5 pages, 4 figures. Oral contrubution. Proceedings of the workshop
"XXI Century challenges for stellar evolution", held in Cefalu' (Sicily,
Italy), August 29 - September 2, 2007; S. Cassisi and M. Salaris Eds. To be
published in Mem. SAIt Vol. 79 No.
The BaSTI Stellar Evolution Database: models for extremely metal-poor and super-metal-rich stellar populations
We present an extension of the BaSTI stellar evolution database to extremely
metal poor () and super-metal-rich (Z=0.05) metallicities,
with both scaled-solar and -enhanced ([/Fe]=0.4) heavy element
distributions. These new tracks (from the pre-main sequence to the
early-asymptotic giant branch phase), horizontal branch models and isochrones,
will enable the use of the BaSTI database to study, i.e., the most metal poor
populations found in Local Group faint dwarf galaxies, and the metal rich
component of the Galactic bulge. An overview of several fundamental predictions
of stellar evolution over the full metallicity range of BaSTI is presented,
together with comparisons with literature calculations at and
Z=0.05.Comment: 11 pages, 15 Figures, A&A in pres
A large stellar evolution database for population synthesis studies: VI. White dwarf cooling sequences
We present a new set of cooling models and isochrones for both H- and
He-atmosphere white dwarfs, incorporating accurate boundary conditions from
detailed model atmosphere calculations, and carbon-oxygen chemical abundance
profiles based on updated stellar evolution calculations from the BaSTI stellar
evolution archive - a theoretical data center for the Virtual Observatory. We
discuss and quantify the uncertainties in the cooling times predicted by the
models, arising from the treatment of mixing during the central H- and
He-burning phases, number of thermal pulses experienced by the progenitors,
progenitor metallicity and the reaction rate. The
largest sources of uncertainty turn out to be related to the treatment of
convection during the last stages of the progenitor central He-burning phase,
and the reaction rate. We compare our new models
to previous calculations performed with the same stellar evolution code, and
discuss their application to the estimate of the age of the solar neighborhood,
and the interpretation of the observed number ratios between H- and
He-atmosphere white dwarfs. The new white dwarf sequences and an extensive set
of white dwarf isochrones that cover a large range of ages and progenitor
metallicities are made publicly available at the official BaSTI website.Comment: 34 pages, 13 figures, The Astrophysical Journal, in pres
Effective temperatures of red giants in the APOKASC catalogue and the mixing length calibration in stellar models
Red giants in the updated APOGEE-Kepler catalogue, with estimates of mass,
chemical composition, surface gravity and effective temperature, have recently
challenged stellar models computed under the standard assumption of solar
calibrated mixing length. In this work, we critically reanalyse this sample of
red giants, adopting our own stellar model calculations. Contrary to previous
results, we find that the disagreement between the effective temperature scale
of red giants and models with solar calibrated mixing length disappears when
considering our models and the APOGEE-Kepler stars with scaled solar metal
distribution. However, a discrepancy shows up when alpha-enhanced stars are
included in the sample. We have found that assuming mass, chemical composition
and effective temperature scale of the APOGEE-Kepler catalogue, stellar models
generally underpredict the change of temperature of red giants caused by
alpha-element enhancements at fixed [Fe/H]. A second important conclusion is
that the choice of the outer boundary conditions employed in model calculations
is critical. Effective temperature differences (metallicity dependent) between
models with solar calibrated mixing length and observations appear for some
choices of the boundary conditions, but this is not a general resultComment: 8 pages, 10 figures, Astronomy & Astrophysics, in pres
Photometric properties of stellar populations in Galactic globular clusters: the role of the Mg-Al anticorrelation
We have computed low-mass stellar models and synthetic spectra for an initial
chemical composition that includes the full C-N, O-Na, and Mg-Al abundance
anticorrelations observed in second generation stars belonging to a number of
massive Galactic globular clusters. This investigation extends a previous study
that has addressed the effect of only the C-N and O-Na anticorrelations, seen
in all globulars observed to date. We find that the impact of Mg-Al abundance
variations at fixed [Fe/H] and Helium abundance is negligible on stellar models
and isochrones (from the main sequence to the tip of the red giant branch) and
bolometric corrections, when compared to the effect of C-N and O-Na variations.
We identify a spectral feature at 490-520 nm, for low-mass main sequence stars,
caused by MgH molecular bands. This feature has a vanishingly small effect on
bolometric corrections for Johnson and Stroemgren filters that cover that
spectral range. However, specific narrow-band filters able to target this
wavelength region can be powerful tools to investigate the Mg-poor unevolved
stars and highlight possible splittings of the MS due to variations of Mg
abundances.Comment: 8 pages, 9 figures, Astronomy & Astrophysics in pres
A large stellar evolution database for population synthesis studies. IV. Integrated properties and spectra
This paper is the 4th in a series describing the latest additions to the
BaSTI stellar evolution database, which consists of a large set of homogeneous
models and tools for population synthesis studies. Here we present a new set of
low and high resolution synthetic spectra based on the BaSTI stellar models,
covering a large range of simple stellar populations (SSPs) for both scaled
solar and alpha-enhanced metal mixtures. This enables a completely consistent
study of the photometric and spectroscopic properties of both resolved and
unresolved stellar populations, and allows us to make detailed tests on their
integrated properties. Our low resolution spectra are suitable for deriving
broadband magnitudes and colors in any photometric system. These spectra cover
the full wavelength range (9-160000nm) and include all evolutionary stages up
to the end of AGB evolution. Our high resolution spectra are suitable for
studying the behaviour of line indices and we have tested them against a large
sample of Galactic globular clusters. We find that the range of ages, iron
abundances [Fe/H], and degree of alpha-enhancement predicted by the models
matches observed values very well. We have also tested the global consistency
of the BaSTI models by making detailed comparisons between ages and
metallicities derived from isochrone fitting to observed CMDs, and from line
index strengths, for the Galactic globular cluster 47Tuc and the open cluster
M67. For 47Tuc we find reasonable agreement between the 2 methods, within the
estimated errors. From the comparison with M67 we find non-negligible effects
on derived line indices caused by statistical fluctuations, which are a result
of the specific method used to populate an isochrone and assign appropriate
spectra to individual stars. (abridged)Comment: 21 pages including 13 figures. Accepted for publication in ApJ. Low
and high resolution integrated spectra, magnitudes, and mass-to-light ratios
will appear on the BaSTI website by 1st November 2008 - see
http://193.204.1.62/index.htm
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